Deep below the surface of the world’s oceans, an immense discovery has upended long-held assumptions about where life exists—and how much of it there really is.

Canadian-led research teams, using a global fleet of 903 autonomous underwater robots, have uncovered a previously invisible reservoir of life: phytoplankton biomass so vast it matches the weight of 250 million elephants.

This network of Biogeochemical-Argo (BGC-Argo) robotic floats didn’t just stumble upon a curious anomaly. It mapped an enormous 314 teragrams of phytoplankton carbon—roughly 346 million tons—across the world’s open oceans.

That mass, much of it hidden far beneath the surface, had gone undetected by satellites for decades. The finding, first detailed by researchers at Dalhousie University and published in the journal PNAS, marks a turning point in our understanding of Earth’s primary producers.

These microscopic organisms help produce nearly half of the planet’s oxygen.

Microscopic Builders With Planet-Sized Impact

Phytoplankton, despite their microscopic size, drive the ocean’s productivity. These photosynthetic organisms float in sunlit waters, absorbing carbon dioxide and pumping out oxygen. While their role as the base of the marine food web is well known, their impact on global climate regulation is still unfolding. Estimates suggest they produce nearly half of the world’s oxygen and fix as much carbon annually as all land plants combined, Nature reports.

Now, thanks to the depth-resolved observations of the BGC-Argo floats, scientists can finally trace where most of that carbon ends up. Roughly half of the total phytoplankton biomass exists below the surface layers that satellites monitor, Interesting Engineering reports. That means a significant portion of the ocean’s carbon cycling has been taking place unnoticed, in a zone previously beyond reach.

Phytoplankton remove vast amounts of carbon dioxide from the atmosphere.

Breaking the Surface Bias

For decades, satellites have relied on chlorophyll-a—a pigment essential for photosynthesis—to estimate phytoplankton abundance. But chlorophyll isn’t a reliable proxy for total biomass, especially at depth. As light fades, phytoplankton increase their chlorophyll levels through photoacclimation, which leads to a false spike in surface-based measurements.

This mismatch between pigment and actual carbon content has masked vast stores of organic matter below, Terra Daily explains.

By diving hundreds of meters deep and collecting nearly 100,000 water-column profiles over a decade, the floats are filling that blind spot. The results show that in two-thirds of the global ocean, the peak timing of biomass does not match what satellites detect. In the tropics, that lag can reach up to 12 weeks, severely skewing models of marine productivity and carbon flux.

Photo: Wikimedia Commons / Jeff Schmaltz, MODIS Land Rapid Response Team at NASA GSFC, License: Public Domain

A phytoplankton bloom off Ireland.

The Biological Pump at Work

What happens to all that phytoplankton? Much of it is consumed in the upper ocean by zooplankton and other creatures. But a significant share sinks. Once it descends beyond 500 meters, it enters the ocean’s biological pump—a process that locks away carbon for centuries. The deeper the material goes, the longer it remains out of the carbon cycle. Only a small portion—about 0.1%—gets permanently buried in sediments and may one day become oil, Nature notes.

Without this pump, atmospheric carbon dioxide levels could be up to 200 parts per million higher. And without phytoplankton, there would be no pump at all.

Photo: Wikimedia Commons / Safa.daneshvar, License: Public Domain

Phytoplankton creates luminescent waves on a beach in Iran.

Why It Matters Now

This discovery is more than a scientific milestone. It’s a warning—and a roadmap. As climate change alters ocean temperature and chemistry, the delicate systems that sustain phytoplankton are already shifting. Warmer waters stratify, cutting off nutrient flow from below. Acidification interferes with shell formation. These pressures may weaken phytoplankton populations, diminishing both their carbon absorption and the food web they support.

Yet, with BGC-Argo data now available at global scale, researchers can better model what comes next. As Jason Deegan reports, integrating robotic and satellite data could improve climate forecasting and inform future policies—including controversial geoengineering proposals meant to increase carbon sequestration.

A Vast World Still Unseen

Phytoplankton are invisible to the naked eye, but they’ve shaped the world for billions of years. They helped create our oxygen-rich atmosphere. They feed nearly all marine life. And they regulate the Earth’s carbon balance with silent persistence.

Now, with 903 robots patrolling the deep, we’re beginning to see just how much we’ve been missing.